It is common practice to inspect work pieces subsequent to production e.g. on a coordinate positioning apparatus, such as a coordinate measuring machine (CMM), in order to check for correctness of predefined object parameters, like dimensions and shape of the object. Moreover, a detection of a surface of an unknown object is of interest in many industrial applications. Such measurement typically also may be provided using a coordinate measuring machine or any other suitable type of scanning device.
In a conventional 3-D coordinate measurement machine, a probe head is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the probe head can be guided to any arbitrary point in space of a measuring volume of the coordinate measuring machine and the object is measurable with a measurement sensor (probing unit) carried by the probe head. Such probing unit can be designed as a tactile probe or an optical sensor providing measurements of surfaces e.g. based on the principle of triangulation.
In a simple form of the machine a suitable transducer mounted parallel to each axis is able to determine the position of the probe head relative to a base of the machine and, therefore, to determine the coordinates of measurement points on the object being illuminated by the sensor. For providing movability of the probe head a typical coordinate measuring machine may comprise a frame structure on which the probe head is arranged and driving means for moving frame components of the frame structure relative to each other.
An advantage of using an optical sensor is that it is not in contact with the part and therefore does not deform it during the measurement or damage it, as may be the case with a tactile probe.
A further advantage of using a (line) triangulation device in combination with a CMM for measuring a surface is the amount of distance information being received by one time step, i.e. distance values along the entire projected triangulation line can be determined and respective coordinates can be derived. Thus, by moving the sensor along a desired measuring path an object to be measured can entirely be scanned significantly faster.
Over the past 20 years, manually operated portable CMM systems (e.g. articulated arm CMMs), comprising typically four segments linked together with one or two rotation axes per linkage and a total of six or seven axes, have become popular for non repetitive measurement tasks on the shop floor. Line triangulation devices are also used on such portable CMMs to greatly increase data capture speed.
Other portable measurement devices where triangulation units are used include optically tracked systems, either using multiple cameras to track the probe location and orientation or interferometric distance tracking devices, where the rotational axes of the probe are tracked using an additional camera.
Other applications for line triangulation sensors include fixed installations where an object is placed in front of the sensor or sensors and single line measurement(s) of the static object are made such that key features of the part can be captured in a single step without the need for expensive positioning systems.
Furthermore, a device for providing a topographic measurement of a surface can be embodied as a (hand-held) device comprising a triangulation sensor, wherein the device is guided along the surface to be measured—either manually or by a robot—and distance data is acquired by the sensor while moving the device. Additionally, the position and/or orientation of such device may continuously be determined (e.g. tracked) in a global coordinate system thus enabling a determination of absolute coordinates corresponding to the object's surface.
In general, triangulation provides a method for scanning a surface in fast and precise manner. Measuring devices working on that principle are for instance known from DE 10 2004 026 090 A1 or WO 2011/000435 A1.
In particular, a line generated by a laser unit, e.g. by moving a laser point along such line or by providing a laser fan, is generated on an object to be measured and the light reflected from the surface is detected by a camera consisting of a light sensitive image sensor (light detector) and electronics to control the image sensor and read out the image. An image of the reflected light is captured and distance information according to the contour of the detected line is derived. Based thereon, topography of the object's surface can be determined.
For triangulation measurements with high precision, an illumination and detection of respectively reflected light has to be provided which comprises a required level of illumination and an adequate detection of the light information. For adjusting illumination so that the reflected light reaches the detector meeting its respective detection properties (e.g. signal-to-noise level and saturation limit) WO 2011/000435 A1 discloses an approach of an in-advanced illumination in order to determine a suitable illumination level for the measuring light. WO 2007/125081 A1 discloses a further approach for actively controlling the power of illuminating light in dependency upon an intensity detected by a camera.
However, in case of regions to be illuminated which significantly differ regarding their reflecting properties there still remains the problem of providing a usable signal over the whole width of a projected laser line. Particularly, surfaces with low roughness, i.e. mirror-like surfaces such as chrome, are difficult to measure due to strong inhomogeneity of the reflected light towards the image sensor.
Another problem with respect to precision of measurements is the possible occurrence of long-term drifts of particular sensor components, e.g. of electronics or optical arrangements, or a drift concerning a relative arrangement of such components. Such drifts may be induced by thermal effects like heating up of a light source or by changing of ambient conditions. As one result of such effect position values which are measured with the system may be acquired with particular errors which finally result in erroneous position measurements.
Possible effects due to thermal changes are unknown tilting or offset of a laser plane which is generated by a laser source on side of a measuring light source. Such tilting or offset can cause significant erroneous measurements for instance regarding calculated distances. Furthermore, a deviation in position and/or orientation may also occur for the light receiving part of the sensor.
Therefore, there remains a problem of providing reliable measuring values in light of above mentioned long term drifts.
Above described problems relate to external influences on the measuring system which result in possible measurement errors. In advance of considering environmental influences there typically has to be provided a further (geometrical) calibration of the triangulation sensor which can guarantee for correct and accurate determination of measuring values. In particular, reference coordinates at an object can be measured with the triangulation sensor and verified or corrected by use of an additional measuring system.
Such basic calibration typically is comparatively time consuming with respect to setting up a calibration arrangement and quite complex as there has to be provided exact reference coordinate information.